The distribution within primates of nucleic acid sequences related to endogenous retroviruses and their sequence organization was studied. Specific goals are (1) to detect and characterize the virus-related sequences in primates and the factors involved in their evolution; (2) to develop probes appropriate for the detection of virus-related sequences in human cells; (3) to characterize the organization of the 50-100 copies of endogenous sequences through restriction mapping and recombinant DNA techniques.